1. Field of the Invention
The present invention relates to a light emitting module and method for shaping a beam emitted by a light source and an optical pickup apparatus employing the same. More particularly, the present invention relates to a light emitting module and method for shaping a beam with a beam shaping window and an optical pickup apparatus employing the same.
2. Description of the Related Art
In general, a light emitting module is employed in an optical pickup apparatus for performing non-contact recording and/or reproducing of information on an optical recording medium and includes a light source that emits a laser beam and a monitor photodetector that monitors the optical output power of the laser beam emitted by the light source.
FIG. 1 is a partially broken away perspective view of a conventional light emitting module, and FIG. 2 is a schematic diagram of a conventional semiconductor laser.
Referring to FIG. 1, a typical light emitting module includes a cap 3 that is disposed on a base 1 and has a window 5 that transmits a laser beam, a mount 2 disposed on the base 1, a semiconductor laser 10 that is disposed on a sidewall of the mount 2 and emits a laser beam of a predetermined wavelength in two directions, and a monitor photodetector 7 that is disposed on the base 1 and receives a beam L2 emitted by the semiconductor laser 10 in the backward direction. The monitor photodetector 7 monitors the optical output power of a laser beam L1 emitted in the forward direction of the semiconductor laser 10 based on a detected signal. The semiconductor laser 10 and the monitor photodetector 7 are connected to leads 9 and used as conducting lines for carrying current.
Referring to FIG. 2, a laser beam emitted at a predetermined position of an active layer 11 of the semiconductor laser 10 has an elliptical cross-section. The elliptical beam profile is due to a difference in the angle that a laser beam emitted in a thickness direction of the active layer 11 and a width direction perpendicular thereto will diffract. The difference in diffraction angles is caused by a difference in the size of beam exits. The laser beams in the thickness (Y-axis) direction and in the width (X-axis) direction appear to have been emitted from the front of the laser 10 and a location that is a distance ΔZ behind the front of the laser 10, respectively. Here, the distance difference ΔZ is called an astigmatism. In FIG. 2, θ∥ and θ⊥denote angles in which the beam diverges in horizontal and vertical directions, respectively.
When the light emitting module is employed in an optical pickup apparatus, astigmatism occurs due to the geometry and structure of a light source. Large amounts of astigmatism results in degradation of jitter characteristics when a signal reflected from an optical recording medium is reproduced through a main photodetector of the optical pickup apparatus. This adversely affects the quality of a detected signal. Here, jitter refers to a displacement or deviation of a signal from its ideal location caused by defects in pits recorded on the optical recording medium or the asymmetric shape of a beam spot created on the optical recording medium.
Since the jitter in particular severely degrades the quality of information being stored on the optical storage medium, there is a need for beam shaping in order to use the light emitting module in an optical pickup apparatus for recording.
As a solution for beam shaping, a beam shaping prism configured as shown in FIG. 3 has been proposed. Referring to FIG. 3, a conventional beam shaping prism 20 includes an incident surface 21 on which a laser beam is incident and which is disposed opposite a semiconductor laser 10 and a reflecting surface 23 that reflects the incident laser beam. The beam shaping prism 20 is optically arranged such that the incident surface 21 is inclined at an angle to the semiconductor laser 10 and refracts a laser beam emitted by the semiconductor laser 10.
The beam emitted by the semiconductor laser 10 has an elliptical cross-section indicated by dotted line A. The elliptical beam is refracted through the incident surface 21, is reflected off the reflecting surface 23, and shaped into a circular profile indicated by dotted line B.
However, use of the conventional beam shaping prism having the above-described configuration requires large assembling tolerances as well as high manufacturing costs due to a complicated optical structure. Furthermore, there is a high possibility that an optical axis will deviate during high-temperature operation.